An insect uses its digestive system to extract nutrients and other substances from the food it consumes. Most of this food is ingested in the form of macromolecules and other complex substances (such as proteins, polysaccharides, fats, nucleic acids, etc.) which must be broken down by catabolic reactions into smaller molecules (i.e. amino acids, simple sugars, etc.) before being used by cells of the body for energy, growth, or reproduction. This break-down process is known as digestion.
All insects have a complete digestive system. This means that food processing occurs within a tube-like enclosure, the alimentary canal, running lengthwise through the body from mouth to anus. Ingested food usually travels in only one direction. This arrangement differs from an incomplete digestive system (found in certain lower invertebrates like hydra and starfish) where a single opening to a pouch-like cavity serves as both mouth and anus. Most biologists regard a complete digestive system as an evolutionary improvement over an incomplete digestive system because it permits functional specialization -- different parts of the system may be specially adapted for various functions of food digestion, nutrient absorption, and waste excretion. In most insects, the alimentary canal is subdivided into three functional regions: foregut (stomodeum), midgut (mesenteron), and hindgut (proctodeum).
In addition to the alimentary canal, insects also have paired salivary glands and salivary reservoirs. These structures usually reside in the thorax (adjacent to the foregut). Salivary ducts lead from the glands to the reservoirs and then forward, through the head, to an opening (the salivarium) behind the hypopharynx. Movements of the mouthparts help mix saliva with food in the buccal cavity.